Technical Field
The present invention relates generally to the field of temperature and humidity control, and more particularly, to simultaneous control of temperature and humidity.
Background Discussion
Many rooms or enclosed spaces contain equipment or objects that are sensitive to both temperature and humidity. For example, museums and other archive facilities, as well as measurement, manufacturing, cleanroom, and lab environments may require maintaining near-constant temperature and humidity levels, since both the values and any fluctuation in these environmental factors has the potential of damaging the contents or detrimentally influencing operations in these spaces. Simultaneous control of both temperature and humidity is complicated by the fact that relative humidity is a function of not only moisture content, but also air temperature.
Direct expansion (DX) air conditioners are useful in small to medium sized buildings and have certain advantages over conventional chilled-water based air conditioning (AC) systems, such as having higher energy efficiency and lower ownership and maintenance costs. DX systems generate conditioned air via a refrigeration cycle using compressors. Liquid refrigerant passes through an expansion device, which is typically a valve, just before entering a cooling coil (an evaporator). The expansion device reduces the pressure and temperature of the refrigerant to the point where it is colder than the air passing through the coil. Cooling is accomplished by blowing air over the cooling coil. DX systems owe their efficiency to the fact that the air used for cooling a conditioned space is directly chilled by the refrigerant in the cooling coil of the air handling unit. As shown in FIG. 1, the components of a DX system typically include an evaporator, a compressor, a condenser, and an expansion device, although any system that uses refrigerant and an evaporator coil can be called a DX system.
DX systems also present difficulties in controlling both temperature and humidity, since in most DX systems the cooling coil must simultaneously perform both cooling and dehumidification functions. For instance, in reference to FIG. 2, an example of a conventional temperature and humidity control system is illustrated, where temperature control is accomplished by either cooling the air using a compressor or heating the air using a heater. Control of the humidity is accomplished by cooling the air to a temperature below the dew point temperature, which is the temperature at which water condenses from air, using a compressor and then either subsequently heating the air by means of a heater, or injecting water vapor using a humidifier. DX systems equipped with a single-speed compressor and supply fan rely on on/off cycling of the compressor for providing an economical, but discontinuous approach to temperature control. This configuration gives priority to temperature control, with control of humidity being secondary. Under these operating conditions, the level of precision for the regulation of humidity is ±7-8%, which is inadequate for many of the applications mentioned above, which may require the precision for humidity control to be within ±5%.
Humidity can be more closely controlled, as shown in FIG. 3, by continuously operating the compressor, which allows the DX device to dehumidify the air, but results in air that is too dry and/or too cool. Therefore, the humidifier operates continuously to humidify the air, and the heater may also be operated more frequently. This configuration leads to increased energy and maintenance costs, since the compressor and humidifier are in constant operation.